Note 9 - Share-based Compensation - Stock Option Activity Under Equity Incentive Plans and Director Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Common Stock [Member]
Exercised, aggregate intrinsic value	$ 25,912	$ 8,280	$ 10,621
Common Stock Options [Member]
Outstanding, number of options (in shares)	1,245,922
Outstanding, weighted average exercise price (in dollars per share)	$ 18.08
Granted, number of options (in shares)	70,471	100,615	116,276
Granted, weighted average exercise price (in dollars per share)	$ 62.23
Exercised, number of options (in shares)	(630,373)
Exercised, weighted average exercise price (in dollars per share)	$ 14.20
Forfeited, number of options (in shares)	(85,449)
Forfeited, weighted average exercise price (in dollars per share)	$ 32.28
Outstanding, number of options (in shares)	600,571	1,245,922
Outstanding, weighted average exercise price (in dollars per share)	$ 25.31	$ 18.08
Outstanding, weighted average remaining contractual term (Year)	5 years 6 months 29 days
Exercisable, number of options (in shares)	270,876
Weighted average exercise price (in dollars per share)	$ 18.00
Exercisable, weighted average remaining contractual term (Year)	4 years 1 month 20 days
Common Stock Options [Member] | Common Stock [Member]
Exercised, aggregate intrinsic value	$ 25,912
Outstanding, aggregate intrinsic value	11,665
Exercisable, aggregate intrinsic value	$ 6,811